C 2008 The Authors
Tellus (2008), 60B, 365–371
C 2008 Blackwell Munksgaard
Journal compilation Printed in Singapore. All rights reserved
TELLUS
Observations on nocturnal growth of atmospheric
clusters
By H E I K K I J U N N I N E N 1 ∗ , M I R A H U L K KO N E N 1 , I L O NA R I I P I N E N 1 , T U O M O N I E M I N E N 1 ,
A N N E H I R S I K KO 1 , TA N JA S U N I 1 , M I C H A E L B OY 1 , S H A N - H U L E E 2 , M A R KO VA NA 1,3 ,
H A N N E S TA M M E T 3 , V E L I - M AT T I K E R M I N E N 4 and M A R K K U K U L M A L A 1 ,
1
Department of Physics, Division of Atmospheric Sciences and Geophysics, PO Box 64, FI-00014 University of
Helsinki, Finland; 2 Department of Chemistry, Kent State University, Kent, Ohio 44242, USA; 3 Department of
Environmental Physics, University of Tartu, 18 Ülikooli St., 50090 Tartu, Estonia; 4 Finnish Meteorological Institute,
Research and Development, Erik Palmenin aukio 1, PO Box 503, FI-00101, Helsinki, Finland
(Manuscript received 25 October 2007; in final form 17 March 2008)
ABSTRACT
In this paper, we summarize recent observations of nighttime nucleation events observed during 4 yr, from 2003 to
2006, at the SMEAR II station in Hyytiälä, southern Finland. Formation of new atmospheric aerosol particles has
been frequently observed all around the world in daytime, but similar observations in nighttime are rare. The recently
developed ion spectrometers enabled us to measure charged aerosol particles and ion clusters to diameters <1 nm and
are efficient tools for evaluating cluster dynamics during nighttime. We observed clear growth of cluster ions during
approximately 60 nights per yr. The newly formed intermediate ions usually persisted for several hours with typical
concentrations of 100–200 cm−3 . The evolution of nighttime growth events is different compared with daytime events.
The mechanism behind nighttime events is still unclear, but the behaviour can be described by the hypothesis of activation
of clusters.
1. Introduction
Formation of new aerosol particles in the atmosphere is a
complicated series of chain reactions, including the production of nanometre-size clusters from precursor vapours, the
growth of these clusters to detectable sizes by condensation
of, for instance, sulphuric acid and organic vapours and the
simultaneous removal of clusters by coagulation with the preexisting aerosol particle population (e.g. Kerminen et al., 2001;
Kulmala, 2003). Once formed, the aerosol particles need to grow
further to sizes above 50–100 nm in diameter to be able to influence climate by scattering solar radiation or by being activated into cloud droplets. At smaller sizes, the particles may
have an direct effect on human health (Ibald-Mulli et al., 2002;
Pope and Dockery, 2006; Weschler et al., 2006) and atmospheric
composition.
Over the past decade or so, aerosol formation has been observed at a large number of sites around the world (Kulmala
et al., 2004a). Such observations have been performed on dif∗ Corresponding author.
e-mail: [email protected]
DOI: 10.1111/j.1600-0889.2008.00356.x
Tellus 60B (2008), 3
ferent platforms (on the ground, on ships, on aircrafts) and over
different time periods (focus campaigns or continuous measurements). In the continental boundary layer, regional nucleation
events are common. Such events increase particle concentrations
over a distance of hundreds of kilometres.
Although there are some atmospheric observations on nighttime new particle formation (NPF) (e.g. Wiedensohler et al.,
1997; Vehkamäki et al., 2004; Lee et al., 2008; Suni et al., 2008),
studies reporting this phenomenon are sparse. On the other hand,
many of the nighttime NPF events may start from the cluster ion
pool, the existence of which has been known for decades (e.g.
Kulmala and Tammet, 2007 and references therein). Recently,
Kulmala et al. (2007b) observed a constant pool of neutral clusters also. Because of their unique capability to detect ions and
charged particles down to <1 nm sizes, the new ion spectrometers, as used in these cited studies, can provide new insight also
to nighttime NPF events.
Here we investigate episodes of nocturnal cluster ion growth
during 2003–2006 in a pine forest in southern Finland, where
clear NPF has been frequently observed during the daytime (e.g.
Mäkelä et al., 1997; Kulmala et al., 2001; Dal Maso et al., 2005,
2007). Our main aim is to examine nighttime NPF events and
discuss their possible connections to day time NPF events.
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H. JUNNINEN ET AL.
2. Material and methods
2.3. NPF event classification
2.1. Site description
First, we plotted the AIS and BSMA data as surface plots to
see the development of particle and cluster size distribution as a
function of time (Fig. 1). To better identify nighttime behaviour,
we plotted two subsequent days in one figure. Since the observed events were very different from corresponding daytime
events, we modified the visual method described by Dal Maso
et al. (2005, 2007) for classification of daytime NPF events.
Besides surface plots, we plotted cluster concentrations in the
BSMA channel 5 (size range 1.3–1.8 nm in Tammet diameter, see
Tammet, 1995) for each classified day. This size class usually
represents the upper size limit of the cluster ion pool. Therefore,
an increase of the ion concentration in this size channel is indicative of an increased average size of cluster ions and therefore the
growth event.
We excluded data that were measured during the malfunctioning or maintenance of the instruments. A nighttime NPF event
was defined as an increase in concentration in the 1.3–1.8 nm
size class and a simultaneous, visually apparent growth of clusters. An increase in the ion concentration alone is not a sufficient
criterion because the mixing volume in the nocturnal boundary
layer (NBL) is smaller than that of the daytime boundary layer.
This would lead to increased nighttime ion concentrations even
if the total number of ions were constant inside the boundary
layer. In addition, an important ion source, radon, accumulates
close to the ground during the night and can, thereby, increase ion
concentrations (e.g. Hirsikko et al., 2007b). For each nighttime
NPF event, we determined the characteristics and duration of the
intermediate ion burst or the growth of the clusters and classified
the events as follows: Class I—‘hump’ shaped formation events;
Class II—‘apple’ shaped formation events and Class III—very
strong events where intermediate ions up to several nanometres
Our measurements took place in Hyytiälä, southern Finland,
at the station for measuring forest ecosystem (SMEAR II,
61˚51 N,24˚17 E, 181 m asl), research site operated by the University of Helsinki. Several continuous measurements have been
performed at SMEAR II. The station is located at a rural site,
surrounded by pine-dominated forests. The station is equipped
with extensive instrumentation for atmospheric measurements
(for a detailed description, see e.g. Kulmala et al., 2001; Hari
and Kulmala, 2005). Precipitation measurements used in this
study were measured above the forest at the height of 18 m,
with precipitation meter FD12P Weather sensor (Vaisala Oyj,
Helsinki, Finland).
2.2. Atmospheric observations with ion spectrometers
We measured size distributions of atmospheric ions and charged
aerosol particles with the balanced scanning mobility analyzer (BSMA; Tammet, 2004) and the air ion spectrometer
(AIS; Mirme et al., 2007), both manufactured by Airel Ltd,
Estonia. The BSMA consists of two plain aspiration-type differential mobility analysers, one for positive and the other
for negative ions. The measured electric mobility range is
0.032–3.2 cm2 V−1 cm−1 . The mobility distribution is converted to a size distribution using an algorithm (Tammet, 1995),
with the resulting size distribution range from 0.4 to 6.3 nm.
The AIS is based on the same principle, but measures particles with diameters from 0.34 to 40 nm. The operation period used in the present study was from 1 April 2003 to
21 December 2006.
50
Number concentration (cm–3)
B)
10
10
5
5
1
1
0.5
0.5
80 C)
200
1000
–3
Diameter (nm)
A)
D)
100
60
150
40
100
20
50
dN/d(logDp) (cm )
50
0
0
00:00 12:00 00:00 12:00 00:00 00:00 12:00 00:00 12:00 00:00
10
Fig. 1. Example of Class I nocturnal NPF
event. (a) Number concentration of negative
air ions measured with AIS on 30 April
2007–1 May 2007. The nocturnal event is
highlighted. (b) Absolute number
concentrations of negative, 1.3–1.8 nm
cluster ions measured with AIS (solid) and
BSMA (dashed). (c) Same as (a), but
measured on 4 June 2004–5 June 2004. (d)
Same as (b), but measured on 4 June 2004–5
June 2004.
Tellus 60B (2008), 3
N O C T U R NA L G ROW T H O F AT M O S P H E R I C C L U S T E R S
50
B)
10
10
5
5
1
1
0.5
0.5
120 C)
350
100
300
80
Diameter (nm)
Number concentration (cm–3)
100
200
60
150
40
100
20
50
0
0
00:00 12:00 00:00 12:00 00:00 00:00 12:00 00:00 12:00 00:00
10
50
A)
B)
10
10
5
5
1
1
0.5
0.5
700 C)
600
300
500
200
400
300
200
100
1000
D)
250
100
150
100
50
0
0
00:00 12:00 00:00 12:00 00:00 00:00 12:00 00:00 12:00 00:00
in size appear suddenly and persist, with no observable growth,
for many hours.
Classes I–III describe the three most common nighttime NPF
types observed from the data set. Class I portrays the most frequent type, a clear ‘shoulder’ above the cluster mode in the surface plot and a clear increase in 1.3–1.8 nm cluster concentration in the BSMA data (Fig. 1). Class II events are characterised
by a roundish-shaped particle burst from 1.3 to approximately
8–10 nm, (Fig. 2). Class III (Fig. 3) events are the strongest
and produce a large number of intermediate ions up to several
nanometres, which persist for many hours and then suddenly
disappear. These events bear some resemblance to the nocturnal
events seen in a Eucalypt forest in Tumbarumba and South-East
Australia (Suni et al., 2008). However, we never observed any
Tellus 60B (2008), 3
D)
250
50
Fig. 3. Example of Class III nocturnal NPF
event. (a) Number concentration of negative
air ions measured with AIS on 7 October
2003–8 November 2007. The nocturnal
event is highlighted. (b) Absolute number
concentrations of negative, 1.3–1.8 nm
cluster ions measured with AIS (solid) and
BSMA (dashed). (c) Same as (a), but
measured on 5 December 2004–6 December
2004. (d) Same as (b), but measured on 5
December 2004–6 December 2004.
1000
dN/d(logDp) (cm–3)
Fig. 2. Example of Class II nocturnal NPF
event. (a) Number concentration of negative
air ions measured with AIS on 27 November
2003–28 November 2007. The nocturnal
event is highlighted. (b) Absolute number
concentrations of negative, 1.3–1.8 nm
cluster ions measured with AIS (solid) and
BSMA (dashed). (c) Same as (a), but
measured on 15 December 2004–16
December 2004. (d) Same as (b), but
measured on 15 December 2004–16
December 2004.
Number concentration (cm–3)
Diameter (nm)
A)
dN/d(logDp) (cm–3)
50
367
10
banana-type events, which are typical for daytime(Dal Maso
et al., 2005).
3. Results and discussion
Table 1 shows the number of nighttime NPF events and nights
with no formation (non-events) in 2003–2006. The most numerous are Class I events: they occurred on approximately 1/6 of the
nights. The length of Class I events varied from 2 to 20 h, with
median length for positive clusters 6.9 h and for negative ones
8.6 h (see Fig. 4).
The annual variation of the nighttime NPF nights was apparent. The event frequency peaked clearly in May, decreased
towards autumn and reached the minimum in winter (Figs. 5a
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H. JUNNINEN ET AL.
Table 2. Co-occurrence of negative and positive nocturnal NPF
events (frequencies)
Table 1. Distribution of classified nights to non-events and events
Year
Negative ions
Positive ions
I
II
I
II
III
50
65
54
73
242
3
5
6
11
25
41
64
51
63
219
0
2
2
0
4
3
1
0
2
6
Non-event Event MD
2003
2004
2005
2006
Total
176
258
246
255
935
69
99
77
99
344
120
9
42
11
182
III
1
3
1
1
6
Three event classes are separated for negative and positive ions. For
non-events, only the night when there is no event in positive or
negative ions are counted. Events are counted if any of charges have
event of any class. MD – missing data.
50
Frequency
40
30
20
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Duration of events [h]
Fig. 4. Durations of nocturnal NPF events. Negative ions are
represented by grey bars and positive ions by black ones.
A)
B)
1 2 3 4 5 6 7 8 9 10 11 12
Month
1 2 3 4 5 6 7 8 9 10 11 12
Month
Frequency
50
40
30
20
10
Fig. 5. Seasonal variation of nocturnal NPF events for years
2003–2006. (a) all event classes (b) Class I events. Negative ions are
represented by grey bars and positive ions by black ones.
and b). Positive ions participated less frequently in nighttime formation in winter and autumn than negative ions. This suggests
that negative ions require lower concentrations of condensing
vapours than positive ones to start growing to larger sizes (see
also Kulmala et al., 2007a). Interestingly, laboratory studies also
show that negative sulphuric acid ion clusters are more stable
Positive ions
MD
Negative ions
MD
Non-event
Class I
Class II
Class III
Total
152
5
0
0
0
157
Non-event Class I Class II Class III Total
25
935
88
23
4
1075
0
67
151
0
1
219
0
2
0
2
0
4
0
2
3
0
1
6
177
1011
242
25
6
1461
MD – missing data.
than positive ones (e.g. Kim et al., 1998; Lovejoy et al., 2004).
Simultaneous nighttime NPF events of both negative and positive clusters occurred in 48% of Class I cases (Table 2). In 28%
of the Class I events, negative clusters grew but positive clusters
did not, whereas in 22% of the events only positive clusters grew.
No Class II or III NPF events were observed at relative humidity <80%, a clear indication that these events require high humidity to take place. The production of intermediate ions during
rainfall, as well as in waterfall experiments, has been observed
in several studies (e.g. Hõrrak et al., 2005; Hirsikko et al., 2007;
Laakso et al., 2006; Parts et al., 2007). Rain explained 22 out of
25 Class II and III nighttime NPF events. Intermediate clusters in
the size range of 2–4 nm appeared when precipitation exceeded
0.1 mm h–1 . First, negative intermediate clusters appeared, and
if rain lasted long enough (longer than 2 h), positive intermediate clusters also became measurable. These clusters disappeared
immediately when the rain stopped.
The three Class II and III events that were not associated
with rain, occurred in winter. During these events, night-time
temperatures suddenly increased above zero degrees and only
negative intermediate clusters appeared. These ions disappeared
shortly after the temperature dropped below zero degrees again
(Fig. 6). A night with an above-zero temperatures occurs only
rarely after a day with subzero temperatures. Therefore, Class
II and III events were very rare during winter. However, during
spring and autumn, the temperature commonly fluctuates around
zero, being negative at night and positive during the day. Formation of ion clusters was observable during such temperature
fluctuations, but this phenomenon took place during daytime
when many other processes associated with radiation, and thus
the expected photochemical reactions, were active. This makes
identifying the underlying causes of cluster formation difficult. A
few exceptional daytime–nighttime temperature increases during the winter provide a possibility to study intermediate cluster
formation in the absence of radiation and photochemistry.
The maximum frequency of nighttime NPF events occurred
two months later (May–June) than that of daytime NPF events
(March–April; see Dal Maso et al., 2005). In addition, the
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Median H SO concentration (cm–3)
12
10
8
3
x 10
6
A)
2.5
2
1.5
4
6
2
4
2
0
1
0.5
00:00
00:00
12:00
00:00
29.Nov 30.Nov 01.Dec
02.Dec
03.Dec
04.Dec
2
Fig. 6. Number concentration of 2–4 nm negatively charged cluster
ions (solid line) and ambient temperature (dashed line). X marks are at
midnight of the following day. Data from year 2005.
seasonal variation of daytime NPF typically has a secondary
peak in August–September. The onset of daytime NPF events in
spring seems to follow, quite closely, the increase of the product
of monoterpene and ozone/OH concentrations (Kulmala et al.,
2004b), but no such connection could be identified for nighttime
events. These observations together suggest that either the particle formation mechanisms or the vapours participating in these
process are slightly different during the day- and nighttime in
Hyytiälä.
To find out a potential link between daytime and nighttime
NPF, we grouped the daytime events and non-events (Dal Maso
et al., 2005) to those that were followed by a nighttime event
and to those that were not. We excluded all unclear days from
this analysis. We found 90 daytime events that were followed
by a nocturnal event and only 28 daytime non-events that were
followed by nocturnal events. Since the number of event and nonevent days is approximately the same in Hyytiälä (Dal Maso et
al., 2007), we can conclude that daytime NPF substantially enhances the probability of NPF during the following night. These
findings suggest that the same condensing vapours that cause
daytime NPF, could also be responsible for nighttime formation.
An alternate explanation is that the general ambient conditions
favouring atmospheric NFP tend to prevail more than 12 h at our
measurement site.
NPF nights always had low SO2 concentration (<0.5 ppb) and
condensation sink (3.3 × 10−3 s−1 vs. 4.6 × 10−3 s−1 during nonevents) indicative of relatively clean air. No clear differences in
the ambient temperature between the event and non-event nights
were seen. Both these features are similar to daytime NPF in
Hyytiälä (Lyubovtseva et al., 2005). However, contrary to what
has been observed during daytime (Sogacheva et al., 2005), the
air mass origin seemed to have no effect on the frequency of
nighttime NPF events (data not shown). The reason for this is
unclear. One possibility is that nighttime events are more local
Tellus 60B (2008), 3
3
x 10
12:00
00:00
6
B)
2.5
2
1.5
4
–4
–6
12:00
Time
–2
Median H SO concentration (cm–3)
–3
Temperature (°C) & Ncluster/10 (cm )
14
369
1
0.5
00:00
12:00
00:00
Time
Fig. 7. Median sulphuric acid concentrations (calculated values 1
March 2003–31 May 2005) during nights with (solid line) and without
(dashed line) an event. (a) negative ion events (b) positive ion events.
than daytime events, being therefore less sensitive to the air mass
origin. Another possibility is the factors controlling nighttime
NPF depend weakly on the air mass type.
The concentration of gaseous sulphuric acid was calculated
using the method by Boy et al. (2005). In this study, we changed
this model to account for nighttime OH-radical production by
including an OH-source from the reaction of monoterpenes with
ozone, based on the on master chemical mechanism (MCM, from
the University of Leeds—http://mcm.leeds.ac.uk/MCM/). The
yield for the reaction of alpha-pinene with ozone is 0.7 and of
beta-pinene, 0.3. For all other terpenes, we used a yield of 0.1.
We found that the calculated sulphuric acid concentrations
were substantially higher, frequently by a factor of two, in the
NPF nights compared with other nights (Fig. 7). This is an indication that sulphuric acid plays a role in nocturnal NPF.
Our recent analyses suggest that the average contribution of
ion-induced nucleation to total NPF is less than 10% during daytime in Hyytiälä (Laakso et al., 2007; Gagné et al., 2008). Here,
we found that ion cluster concentrations were clearly higher during event nights compared with non-event nights (Fig. 8). Based
on this, it might be tempting to conclude that ions are central
to nighttime NPF. One should, however, remember that a significant fraction of neutral clusters become charged during their
growth, especially if these clusters grow slowly and were surrounded by many ion clusters (Kerminen et al., 2007). As a result,
additional information on the concentrations of neutral clusters
during nighttime NPF is necessary before we can state anything
370
H. JUNNINEN ET AL.
1000
950
Concentration (cm–3)
900
850
800
750
700
650
600
550
500
00:00
12:00
00:00
Time
12:00
00:00
Fig. 8. Median total concentration of ion clusters (<1.3 nm). Solid
lines are for nights with nocturnal NPF event and dashed lines for
nights without an event. Black lines are for negative ions and grey
lines, for positive ions.
conclusive regarding the involvement of ions or ion-induced nucleation during these nocturnal events.
One major difference between the daytime and nighttime NPF
events was that, in nighttime, the clusters never grew to sizes
larger than a few nanometres. This can result from a combination
of two things. The first is the lower concentrations of condensing
vapours during nighttime, which lead to slower particle growth
rates. The second is the more efficient coagulation scavenging of
the small clusters by the pre-existing particle population; when
clusters grow at a slower rate, a larger fraction of them will be
scavenged before reaching larger sizes (Kerminen et al., 2001).
both day- and nighttime processes. However, the differences in
seasonal patterns of day- and nighttime formation indicate the
opposite. Daytime event frequency had a maximum during the
spring recovery of the forest in March–April and a secondary
peak in the autumn, whereas the nighttime events peaked during
May–June and decreased sharply after that. The role of organic
vapours in nighttime formation, therefore, remains unclear. Regardless of the detailed formation mechanism, the above observations are in line with the hypothesis that atmospheric NPF
might actually be a two-step process (Kulmala et al., 2000), in
which, the first step is the formation of clusters and the second
step is the activation of clusters by sulphuric acid and/or organic
vapours (Kulmala et al., 2006). The first step can also include the
recombination of atmospheric ion clusters into neutral ones. The
growth of clusters observed here is probably the starting point
of the activation process.
The similarities and differences between daytime and nighttime events give us new insights for finding out the reasons for
atmospheric nucleation and initial steps of cluster growth. In
particular, the warm nights during the winter provide a means
to investigate the formation of intermediate ion clusters without
the involvement of radiation and photochemistry. On the other
hand, the growth events of cluster ions might reveal information
on why particles do not always grow into larger sizes. However,
future studies are required to understand the possible mechanisms related to cluster growth and atmospheric nucleation.
5. Acknowledgments
This work was supported by the European Commission and the
Academy of Finland.
4. Conclusions
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at the same time in both negative and positive polarities (151
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